1. Technical Field
The present invention relates generally to a method of manufacturing a hollow cylinder with a bottom, and more particularly to a method of pressing a circular plate to form a hollow cylinder with a bottom which may be employed in rotary head drums of magnetic recording and reproduction systems such as video tape recorders and digital audio tape recorders.
2. Background of Related Art
As is well known in the art, a rotary head type magnetic recording and production system wraps a traveling magnetic tape helically about a rotary head drum to form slant tracks on the magnetic tape for recording information thereon or to reproduce information recorded in slant tracks on the magnetic tape. The rotary head drum usually consists of a bottomed lower cylinder and a rotary upper cylinder disposed coaxially with the lower cylinder.
Such upper and lower cylinders are commonly manufactured by die-casting an aluminum alloy and machining it into a desired shape.
Japanese Patent First Publication No. 63-211113 teaches an alternative manufacturing method of pressing a single disc plate to form a cylindrical member for rotary head drums.
Finishing a casting into a desired shape precisely, however, requires a large amount of machining or a large amount of time for shaping. Within an object die-casted, porosity or cavities may form and appear at a surface of the object after being cut into a final shape, resulting in a defective product. Even if the cavities do not appear at the surface of the object, they may result in rotational unbalance of, for example, a rotary head drum needed to be rotated with high accuracy. An unbalance-correcting operation thus becomes necessary.
The above problems involved in die-casting is not encountered in making a rotary head drum of a single metal plate in press working as taught in the above publication. This press working, however, simultaneously performs three steps of extending the periphery of a disc plate vertically, making the center thereof hollow, forming mounting holes for rotary heads and a rotational shaft, which may result in stress on the finished cylinder, leading to the fracture ultimately. The circularity of a side wall of the cylinder is usually low, causing the center of gravity of the rotary head drum to be shifted from the center of rotation, resulting in rotational shake of the rotary head drum. It is also difficult for the press working to provide a machining allowance to the side wall of the cylinder.